In a conventional telephone system, a central office provides a ring signal to a telephone set to indicate an incoming call. In response to the ring signal, the telephone set produces intermittent, audible tones. A typical ring signal is powered by a -48 VDC central office power supply, and is delivered as a sequence of 90 Vrms, 20 Hz, signals across the two electrical connections, a "ring" and a "tip," extending from a central office to a telephone handset. This local ring signal must be generated even where digital call signaling is used because telephone sets in a local loop may be designed to respond to the characteristic ring signal. Further, the ring signal is used by some telephone sets to directly drive an electro-mechanical ringing device, which places load requirements on the ring signal generator.
One approach to providing a ring signal is to employ a conventional class D switching power amplifier. These amplifiers are efficient; however, they typically require four switching transistors selectively driven by a pulse-width modulated error signal and two transformers. They are thus relatively large and expensive. Additionally, these amplifiers generally require significant output ripple filtering which adds to cost and complexity. The output filter prevents these amplifiers from passing direct current (DC) signals, which presents a further disadvantage where DC signals are desired.
More specialized circuits have been developed specifically for the generation of analog ring signals from digital inputs. For example, U.S. Pat. No. 5,555,301 describes a circuit for converting digital call signaling. However, this circuit is limited to a uni-polar, approximately square ring signal. Another ring signal generator is described in the data sheet for Lucent Technologies' L7590 Telephone Ringing Driver. This circuit receives as an input a 100 kHz pulse-width modulated signal, which is converted to a ring signal. The L7590 requires two power supplies and an external pulse-width modulator. Further, the maximum output excursion from ground is limited by the negative voltage supplied to the circuit.
There is thus a continuing need for a simple ring signal generator which operates from a single power supply and provides voltage gain and power amplification for an analog ring signal.